1. Field of the Invention
The invention relates to a gas diffusion electrode for a polymer electrolyte membrane fuel cell to be operated with a burnable gas and an oxygen-containing gas and comprising an anode electrode, a cathode electrode and a polymer electrolyte membrane disposed therebetween, a polymer electrolyte membrane fuel cell having at least one such gas diffusion electrode, and a method of operating a polymer electrolyte membrane fuel cell.
2. Description of the Related Art
Polymer electrolyte membrane fuel cells contain an anode electrode, a cathode electrode and an ion exchange membrane disposed therebetween. A plurality of fuel cells forms a fuel cell stack, with the individual fuel cells being separated from one another by bipolar plates acting as current collectors. For generating electricity, a burnable gas, e.g. hydrogen, is introduced into the anode region, and an oxidizing agent, e.g. air or oxygen, is introduced into the cathode region. Anode and cathode, in the regions in contact with the polymer electrolyte membrane, each contain a catalyst layer. Alternatively, the catalyst layers may also be applied to the surface of the polymer electrolyte membrane in contact with the anode electrode and the cathode electrode, respectively. In the anode catalyst layer, the fuel is oxidized thereby forming cations and free electrons, and in the cathode catalyst layer, the oxidizing agent is reduced by taking up electrons. The cations migrate through the ion exchange membrane to the cathode and react with the reduced oxidizing agent, thereby forming water when hydrogen is used as burnable gas and oxygen is used as oxidizing agent.
The function of the gas diffusion electrodes consists mainly in discharging the current produced to the current collectors and to allow the reaction gases to diffuse through to the catalytically active layers. The electrodes thus must be electrically conductive and have sufficient diffusion capacity for the reaction gases. Preferably, the electrodes should be hydrophobic at least in the regions facing the membrane, in order to prevent water formed during the reaction from flooding the pores of the electrodes.
In the reaction of burnable gas and oxidizing agent, heat is released which causes evaporation of the water present in electrodes and membrane. The vapor is discharged along with the oxidizing agent stream. This evaporation, on the one hand, causes an indeed desirable cooling of the fuel cell, but on the other hand results in gradual depletion of moisture in the fuel cell. When too much moisture can leak out through the porous electrodes, the moisture content of the polymer electrolyte membrane decreases. The conductivity of the membrane is strongly dependent on its water content. Reduction of the humidity content of the polymer electrolyte membrane has the result that its internal resistance increases, i.e. its conductivity decreases. However, this causes also the performance of the fuel cell to decrease. Efficient operation of a polymer electrolyte membrane fuel cell thus necessitates that the membrane at all times have sufficient moisture at the particular operating conditions (temperature, load) . For this reason, it is necessary in fuel cells with conventional gas diffusion electrodes to supply thereto water in the form of vapor or liquid during operation of the fuel cell. In some embodiments, the supply of membrane humidifying water takes place at the same time with the supply of cooling water, in other embodiments there is provided a separate supply. Care has to be taken that exactly the correct amount of membrane humidifying water is supplied at all times, since a too small amount leads to gradual drying out of the membrane, whereas a too great amount of water supplied results in flooding of the electrodes. It is thus required during operation of the fuel cells to constantly, or at least in short regular intervals, ascertain the moisture content of the membrane and, if necessary, to supply water. This necessitates an additional, external humidifying system subjecting the fuel cells to additional weight and causing additional costs. Up to one third of the weight and costs of a fuel cell stack with conventional electrodes are due to the external humidifying system.
It is the object of the present invention to make available a gas diffusion electrode and, respectively, a polymer electrolyte membrane fuel cell comprising gas diffusion electrode, which allows sufficient membrane moisture to be maintained with continuous operation of the fuel cell under unchanged operating conditions on the average, without water being added for membrane humidification.
Furthermore, it is an object of the invention to make available a method of operating a polymer electrolyte membrane fuel cell in which sufficient membrane moisture is maintained without membrane humidifying water being added.
The object is met by the gas diffusion electrode according to claim 1, the polymer electrolyte membrane fuel cell according to claim 21 and the method of operating a polymer electrolyte membrane fuel cell according to claim 22. Advantageous developments of the invention are indicated in the respective dependent claims.